Kinetics of chlorine oxide radicals using modulated photolysis. Part 2.—ClO and ClOO radical kinetics in the photolysis of Cl2+ O2+ N2 mixtures

Abstract

The low intensity photolysis of Cl₂+ O₂+ N₂ mixtures has been reinvestigated using molecular modulation spectrometry to observe the radical species ClOO and ClO in the ultraviolet region. The results substantially confirm the work of Johnston and coworkers on this system.

The observed kinetic behaviour of ClOO together with kinetic and thermodynamic data from elsewhere lead to a more definitive value for the equilibrium constant K^*₂(in cm³ molecule units): Cl + O₂+ [graphic omitted] ClOO + M; K^*₂=k₂/k_–2= 3.7 × 10^–28Texp(3217/T). The value of K^*₂ allowed rate constants for the following reactions of ClOO at 298 K to be evaluated: Cl + ClOO = Cl₂+ O₂k₃= 9.8 × 10^–11 cm³ molecule^–1 s^–1, Cl + ClOO = 2ClO k₄= 4.7 × 10^–12 cm³ molecule^–1 s^–1. The uncertainty in these values is ≈±50%.

The overall decay rate constant for the disproportionation of ClO radicals exhibited a strong pressure dependence in the range 25–760 Torr and at high pressures gross deviations from the expected second order kinetic behaviour was observed. The pressure dependence of ClO decay was interpreted in terms of a Cl-atom catalysed mechanism involving the Cl₂O₂ molecule: ClO + ClO + M ⇌ Cl₂O₂+ M K^*₆=k₆/k_–6, Cl + Cl₂O₂= ClOO + Cl₂k₇. K^*₆ was estimated as 1.7 × 10^–29T exp (8163/T) cm³ molecule^–1 and k₇≈ 1 × 10^–11 cm³ molecule^–1 s^–1.